1. Field of the Invention
The present invention relates to an illuminating device for providing a substantially coherent illumination beam having a spatial intensity distribution adjustable at will.
2. Description of the Prior Art
A laser beam generally has a gaussian distribution of intensity with circular symmetry in accordance with the section due to the fact that only the fundamental mode resonates in the cavity. In an illuminating device for illuminating an object, for example in a pattern-projection system, the beam from the laser is made wider and divergent. It keeps its gaussian distribution so that the center of the object is illuminated more intensely than the edges. Now, to obtain a good image quality and also line widths identical in the whole field of the projection lens, particularly when the patterns are projected on a photosensitive resin, good uniformity of illumination is required in the image plane. Thus, in the case of production of integrated circuits by direct photorepetition, which comprises formation of a superimposition of levels in a semiconductor substrate, with successive maskings, the finest lines have a width of the order of a micron. Line width dispersion must be less than 10%, which implies illumination intensity variations less than 3%.
Different methods have been proposed for making an illumination uniform: the simplest method consists in widening the beam sufficiently so as to use only the center rays. This method, in order to be practical, has an energy efficiency of the order of 1%, which cancels out a great deal of the interest in using a laser. Other methods using generally the dispersion or the absorption of the beam also have a very low light efficiency. Some methods do not however have this disadvantage. A numerical hologram can be used traced by computer and approximating by a binary function the complex transformation giving a uniform spatial distribution from a gaussian distribution. This method is complex and costly to put into operation. Allowance may also be made for the fact that the gaussian distribution is due to the fact that the laser is monomode, and mirrors having a higher radius of curvature used for the laser, which allows other modes to resonate. An intensity distribution is obtained having a less marked maximum, but the illumination cannot be made sufficiently uniform by this means.